EP1176217B1 - Tôle d' acier à haute résistance laminé à chaud ayant une déformabilité de bordage par étirage excellente et son procédé de fabrication - Google Patents

Tôle d' acier à haute résistance laminé à chaud ayant une déformabilité de bordage par étirage excellente et son procédé de fabrication Download PDF

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Publication number
EP1176217B1
EP1176217B1 EP01306228A EP01306228A EP1176217B1 EP 1176217 B1 EP1176217 B1 EP 1176217B1 EP 01306228 A EP01306228 A EP 01306228A EP 01306228 A EP01306228 A EP 01306228A EP 1176217 B1 EP1176217 B1 EP 1176217B1
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Prior art keywords
mass
steel sheet
rolled steel
hot
ferrite
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EP01306228A
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German (de)
English (en)
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EP1176217A3 (fr
EP1176217A2 (fr
Inventor
Takahiro Kakogawa Works Kobe Steel Ltd Kashima
Shunichi Kakogawa Works Hashimoto
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Kobe Steel Ltd
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Kobe Steel Ltd
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Classifications

    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium

Definitions

  • the present invention relates to a high-strength hot-rolled steel sheet superior in stretch flange formability and a method for production thereof, said steel sheet being suitable for use as a raw material for automotive parts such as chassis and suspension systems (including arms and members).
  • Ti-containing hot-rolled steel sheets have high strength and good workability as disclosed in Japanese Patent Laid-open Nos. 88620/1978 , 106861/1999 and JP02008349 and Japanese Patent Publication Nos. 4450/1987 , 66367/1988 , and 110418/1992 .
  • these disclosures are not concerned at all with the structure desirable for improved stretch flange formability.
  • the present invention was completed to address the above-mentioned problems. It is an object of the present invention to provide a hot-rolled steel sheet having high strength as well as good stretch flange formability. It is another object of the present invention to provide a method for producing the hot-rolled steel sheet.
  • the present inventors found that a hot-rolled steel sheet exhibits good stretch flange formability without its high strength being impaired if it contains 0.10-0.30% of Ti and does not substantially contain the second phase (such as martensite and bainite resulting from transformation at low temperatures) except for ferrite and has a single-phase structure of ferrite with a controlled grain size and shape.
  • the present invention is based on this finding.
  • the gist of the present invention resides in a high stretch hot-rolled steel sheet of claim 1 superior in stretch flange formability which consists of C (0.01-0.10 mass%), Si (present up to no more than 1.0 mass%), Mn (more than 0.5% and no more than 2.5 mass%), P (no more than 0.08 mass%), S (no more than 0.005 mass%), Al (0.015-0.050 mass%), and Ti (0.10-0.30 mass%), and optionally at least one of Nb in an amount not more than 0.4 mass%, B in an amount not more than 0.0010 mass%, and Ca in an amount not more than 0.01 mass%, with the remainder being Fe and inevitable impurities, said hot-rolled steel sheet having a structure consists of ferrite with a second phase in an amount of less than 5% in terms of area ratio, wherein the unit grain has an average particle diameter (d) no larger than 5 ⁇ m, said unit grain being defined such that adjacent grains which surround said unit grain differ from solid unit grain in orientation more than 15° and
  • the gist of the present invention resides also in a method of producing a high-strength hot-rolled steel sheet according to claim 4, said method comprising the steps of heating and hot-rolling a steel sheet having the above-mentioned composition and coiling the hot-rolled steel sheet in such a way that the reduction is no less than 70% at the rolling temperature of 900-840°C and the coiling temperature is 300-500°C or 600-750°C.
  • the requirement for L/d no smaller than 4.0 is met when the reduction is no less than 50%, and hence the resulting steel sheet has good stretch flange formability.
  • the hot-rolled steel sheet according to the present invention exhibits good stretch flange formability without its high strength being impaired owing to its specific composition in which ferrite accounts for a major portion, with a Ti content being 0.10-0.30%, and also owing to its specific structure in which the ferrite unit grain has a specific particle diameter or peripheral shape to prevent crack propagation.
  • the method of the present invention permits easy production of said high-strength hot-rolled steel sheet.
  • the high-strength hot-rolled steel sheet of the present invention should have the above-mentioned specific chemical composition for the reasons given below. ("%” means “mass%”.)
  • C is an essential element to improve strength. C in excess of 0.10% tends to form the second phase structure. Therefore, the lower limit of the C content should be 0.01%, preferably 0.02%, and the upper limit of the C content should be 0.10%, preferably 0.08%.
  • Si is an element to effectively increase the steel strength without deteriorating the steel ductility appreciably, although, if added in a large amount, it causes surface defects including scale defects and promotes generation of coarse ferrite grains which decreases L/d.
  • the upper limit of the Si content should be 1.0%, preferably 0.8%.
  • Mn more than 0.5% and no more than 2.5%
  • Mn is an element that contributes to solid-solution strengthening and in turn imparts strength to steel. It also promotes transformation, thereby forming granular bainitic ferrite and bainitic ferrite. It changes the shape of the grain boundary. It is added in an amount more than 0.5%; however, Mn added in an excess amount results in excessive hardenability, which leads to a large amount of transformation products detrimental to high stretch flange formability.
  • the upper limit of the Mn content is 2.5%, preferably 2.0%.
  • P is an element that contributes to solid-solution strengthening without deteriorating ductility.
  • P added in an excess amount raises the transition temperature after working. Therefore, the content of P should be no more than 0.08%.
  • S forms sulfides (such as MnS) and inclusions detrimental to stretch flange formability.
  • the content of S should be no more than 0.005%. The smaller, the better.
  • Al is added as a deoxidizer. It produces little deoxdizing effect and promotes generation of non-metallic inclusions such as TiN by leaving much N, if its content is less than 0.015%. It forms non-metallic inclusions, such as Al 2 O 3 , detrimental to cleanliness if its content exceeds 0.050%.
  • the content of Al should be 0.015-0.050%.
  • Ti improves hardenability and changes the particle diameter, thereby improving the stretch flange formability.
  • the content of Ti should be no less than 0.10%, preferably no less than 0.20%, and should be no more than 0.30%, preferably no more than 0.25%. Excessive Ti is wasted without additional effects.
  • Ti expands the unrecrystallized austenite region (as mentioned later) and accumulates the deformation strain energy which gives rise to fine grains and also to grains having zigzag grain boundaries both effective for stretch flange formability. This effect is produced most effectively when Ti is added. This effect is not produced when only Nb is added. If Ti content is too small, generation of ferrite is promoted and the zigzag boundaries are not obtained.
  • the high-strength hot-rolled steel sheet of the present invention is composed of the above-mentioned components, with the remainder being substantially Fe. It may contain, in addition to inevitable impurities, one or more of the following elements in an amount not harmful to the effect of the above-mentioned components.
  • Nb should be no more than 0.40%, preferably no more than 0.30%, and the content of B should be no more than 0.0010%, preferably no more than 0.0005%. Excessive Nb and B are wasted without additional effects.
  • Ca reduces MnS detrimental to stretch flange formability and converts it into spherical sulfide (CaS) which is harmless to stretch flange formability.
  • the content of Ca should be no more than 0.01%. Excessive Ca is wasted without additional effects.
  • the hot-rolled steel sheet of the present invention is characterized by its structure as explained below.
  • the steel sheet of the present invention consists of ferrite with a second phase in an amount of less than 5% in terms of area ratio. It should not contain a second phase (such as martensite and bainite resulting from transformation at low temperatures), because ferrite differs in hardness from such a second phase and this difference gives rise to voids and cracks which deteriorate the stretch flange formability.
  • the ferrite includes not only polygonal ferrite structure but also granular bainitic ferrite structure and bainitic ferrite structure. The typical form of these ferrites is known from "Collection of photographs of steel bainite (part 1)" issued by The Iron and Steel Institute of Japan, Fundamental Research Group. All the ferrite structure mentioned above should preferably be a single phase of ferrite. However, it may practically contain a second phase in an amount less than 5% (in terms of area ratio) with only little adverse effect on the stretch flange formability.
  • Ferrite seriously affects plastic deformation and hence stretch flange formability depending on its particle diameter and its grain boundary shape in the structure.
  • Irregular (or zigzag) grain boundaries provide greater boundary strength than straight or flat grain boundaries and hence effectively prevent boundary cracking at the time of deformation.
  • the ferrite structure in the hot-rolled steel sheet be composed of unit grains having an average particle diameter (d) no larger than 5 ⁇ m, wherein all adjacent grains which surround said unit grain differ from said solid unit grain in orientation more than 15°.
  • d average particle diameter
  • ferrite grains do not effectively prevent crack propagation and hence do not contribute to stretch flange formability.
  • stretch flange formability not only is it necessary that unit particles be fine but it is also necessary that each unit grain adjoins its surrounding grains along a boundary whose average length (L ⁇ m) is such that L/d is no smaller than 4.0.
  • any unit grain surrounded by grains such that all adjacent grains differ in orientation less than 15° may be regarded substantially as a single grain from the standpoint of preventing crack propagation. Grain boundaries between grains which differ in orientation less than 15° provide little effect on crack propagation.
  • the particle diameter and boundary length of the unit grain can be determined by EBSP (Electron Back Scattering Pattern) method for measuring the crystal orientation on an etched steel surface. (Measurements are carried out under the condition of 2000 magnifications and 100 steps for 10 ⁇ m.) Measurements give a map showing a grain surrounded by grains all of which have an orientation difference larger than 15°. This map is finally examined by image analysis.
  • EBSP Electro Back Scattering Pattern
  • average particle diameter means an average value of the diameters of imaginary circles each having an area equal to that of a unit grain surrounded by grains all of which have an orientation difference larger than 15°.
  • the high-strength hot-rolled steel sheet is produced by preparing a steel containing the above-mentioned components, heating and hot-rolling the steel slab and coiling the hot-rolled steel sheet in such a way that the reduction is no less than 70% at the rolling temperature of 900-840°C and the coiling temperature is 300-500°C or 600-750°C.
  • the slab should be heated at about 1150-1300°C so that Ti is completely dissolved to form solid solution.
  • the hot-rolled steel sheet should be cooled to the specified temperature at a rate no smaller than 60°C/s, preferably no smaller than 80°C/s, so that ferrite is not generated.
  • the rolling at 900°C or below in finish rolling that follows rough rolling, is usually carried out in the unrecrystallized austenite region in which recrystallization does not take place in the austenite region (or gamma region).
  • Rolling with a reduction no less than 70% in this temperature range imparts sufficient deformation strain to the unrecrystallized austenite.
  • the rolling temperature is no higher than 840°C, the resulting steel sheet consists of two phases (ferrite and gamma regions) and hence is poor in stretch flange formability due to the presence of ferrite worked structure. For this reason, it is necessary that the reduction be no less than 70% at 900-840°C.
  • finish rolling or rough rolling at temperatures exceeding 900°C is not specifically restricted because at such high temperatures the structure undergoes recrystallization which only imparts little deformation strain.
  • finish rolling at temperature exceeding 900°C due to rolling in recrystallization region, coarse ferrite grains occur and desired L/d can not be obtained.
  • the hot-rolled steel sheet composed of unrecrystallized austenite is coiled at a specific temperature (mentioned later) so that fine ferrite grains differing in crystal orientation occurs rapidly during coiling.
  • the ferrite unit grains in the hot-rolled steel sheet have an average particle diameter no larger than 5 ⁇ m.
  • rolling does not cause the unrecrystallized austenite to accumulate sufficient strain energy, with the result that ferrite nucleating sites are limited in number, ferrite nucleation is slow, coarse ferrite grains occur, and ferrite unit grains are outside the prescribed size.
  • the reduction should preferably be no less than 80%.
  • the steel sheet rolled with such a high reduction permits ferrite transformation to take place rapidly during coiling, with the resulting ferrite grains having an irregular grain boundary so that the value of L/d is no less than 4.0.
  • the mechanism by which the crystal boundary becomes irregular is not yet elucidated; however, the present inventors observed that crystal grains became uneven and hence crystal boundaries became irregular when a steel incorporated with a certain an amount of Ti was rolled with a high reduction. This observation suggests the possibility of Ti playing an important role.
  • the coiling temperature should be 300-500°C (preferably 320-480°C) or 600-750°C (preferably 620-720°C). Coiling at a temperature lower than 300°C permits the second phase (such as martensite) to occur easily. By contrast, coiling at a temperature higher than 750°C permits ferrite grains to grow to such an extent that the ferrite unit grain is larger than 5 ⁇ m. Coiling at temperatures higher than 500°C and lower than 600°C should be avoided because it permits the coherent precipitation of TiC on the matrix, which deteriorates the elongation and the stretch flange formability. The lower the coiling temperature or the higher the reduction in the unrecrystallized austenite region, the more effectively the ferrite crystal grains become fine.
  • Table 1 Steels having the chemical composition shown in Table 1 were prepared.
  • the slab of each steel was heated at 1250°C for 30 minutes.
  • the heated slab underwent rough rolling and finish rolling.
  • Table 2 shows the temperature at which finish rolling was started (FET), the temperature at which finish rolling was completed (FDT), and the reduction (R) in finish rolling.
  • FET temperature at which finish rolling was started
  • FDT temperature at which finish rolling was completed
  • R reduction
  • the rolled steel sheet was cooled with mist (at a cooling rate of 65°C/s) and finally coiled at the coiling temperature (CT) shown in Table 2.
  • CT coiling temperature
  • Test specimens conforming to JIS No. 5 were taken from the hot-rolled steel sheets. They were tested for tensile strength (TS) in the rolling direction. They were also tested for stretch flange formability by hole expansion.
  • Specimens for structure observation were taken from the rolled steel sheets. They were examined under an SEM to identify the kind of structure and to calculate the ratio of ferrite area. They were also examined by EBSP method to make a crystal orientation map. Unit grains whose orientation difference is smaller than 15° were measured for particle diameter (d 0 ) and grain boundary length (L 0 ). The average value (d) of d 0 and the average value (L/d) of L 0 /d 0 were calculated. The results are shown in Table 2. Incidentally, the ferrite structure in Table 2 is identified by pF (polygonal ferrite) and bF (bainitic ferrite). Those samples numbered 10, 24, and 34 are identical but are given different numbers for data arrangement.
  • samples Nos. 1, 4, 8, 36, 37, 40, 41, and 44 which were prepared from steels Nos. 1, 4, 8, 9, 10, 13, 14, and 17 each composed of components not conforming to the present invention, are remarkably poor in tensile strength TS or ⁇ .
  • sample No. 1 is characterized by the structure not dominated by ferrite (with 10% martensite) owing to its high c content, and hence it has a very low value of ⁇ .
  • Sample No. 21 is characterized by coarse grains (with a large value of d) owing to the high coiling temperature.
  • Sample No. 23 is characterized by the precipitation of TiC and the low value of ⁇ owing to the inadequate coiling temperature.
  • Sample No. 31 and 32 are characterized by coarse grains (with a large value of d) and a low value of ⁇ owing to an excessively low reduction in the unrecrystallized austenite region despite the adequate coiling temperature.
  • Sample No. 36 is characterized by a low value of L/d owing to a high Si content which promotes ferrite formation.
  • Sample No. 37 is characterized by a low value of L/d owing to a low Ti content which promotes ferrite formation.
  • Sample No. 40 is characterized by a low value of ⁇ owing to a high Ti content which leads to a large amount of Tio and TiN inclusion.
  • Sample No. 41 is characterized by a low value of X owing to a high Al content which leads to a large amount of TiN inclusion.
  • Sample No. 44 is characterized by a low value of ⁇ owing to a high Al content which leads to a large amount of Al 2 O 3 inclusion.
  • Sample No. 48 is characterized by a high value of FET, a large value of d, and a low value of X.
  • Sample No. 49 is characterized by a low value of FDT, worked structure, and a low value of ⁇ .
  • Sample No. 50 is characterized by a low value of CT and a low value of L/d.
  • Sample No. 52 is characterized by a low value of CR, a large value of d, and a low value of ⁇ .
  • those samples (indicated by asterisked sample numbers) satisfying the requirements of the present invention have a high strength (570 N/mm 2 or above), a high value of ⁇ (60% or above), and good stretch flange formability.
  • those samples (Nos. 9, 10, 13, 22, 24, 34 and 35) which are characterized by d lower than 5 ⁇ m and L/d higher than 4.0 have a value of ⁇ higher than 90% and a value of TS ⁇ ⁇ higher than 70000 N/mm 2 ⁇ %, and they are also superior in strength and stretch flange formability.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
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  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Claims (4)

  1. Tôle d'acier laminée à chaud constitué de :
    C : 0,01-0,10 % en masse ;
    Si : présent jusqu'à un maximum de 1,0 % en masse ;
    Mn : plus de 0,5 % et pas plus de 2,5 % en masse ;
    P : pas plus de 0,08 % en masse ;
    S : pas plus de 0,005 % en masse ;
    Al : 0,015-0,050 % en masse ; et
    Ti : 0,10-0,30 % en masse,
    et éventuellement
    Nb : pas plus de 0,40 % en masse ; et/ou
    B : pas plus de 0,0010 % en masse ; et/ou
    Ca : pas plus de 0,01 % en masse ;
    le reste étant du Fe et des impuretés inévitables,
    ladite tôle d'acier laminée à chaud ayant une structure composée de ferrite avec une deuxième phase en une quantité inférieure à 5 % en termes de rapport de surface, dans laquelle le grain élémentaire a un diamètre de particule moyen (d) ne dépassant pas 5 µm, ladite unité étant définie de telle sorte que les grains adjacents qui entourent ledit grain élémentaire diffèrent dudit grain élémentaire en orientation de plus de 15°, et dans laquelle ledit grain élémentaire est contigu aux grains avoisinantes le long d'un joint dont la longueur moyenne (L) est telle que le rapport L/d n'est pas inférieur à 4,0.
  2. Tôle d'acier laminée à chaud selon la revendication 1, comprenant en outre au moins un de Nb dans une quantité ne dépassant pas 0,40 % en masse et de B dans une quantité ne dépassant pas 0,0010 % en masse.
  3. Tôle d'acier laminée à chaud selon l'une quelconque des revendications 1 ou 2, comprenant en outre du Ca dans une quantité ne dépassant pas 0,001 % en masse.
  4. Procédé de production d'une tôle d'acier laminée à chaud selon l'une quelconque des revendications 1 à 3, comprenant les étapes de chauffage, de laminage, de refroidissement et de bobinage dans les conditions suivantes :
    température de chauffage : 1 150-1 300 °C ;
    réduction lors du laminage à 900-840 °C : pas inférieure à 70 % ;
    vitesse de refroidissement : pas inférieure à 60 °C/s ;
    température de bobinage : 300-500 °C ou 600-750 °C.
EP01306228A 2000-07-24 2001-07-19 Tôle d' acier à haute résistance laminé à chaud ayant une déformabilité de bordage par étirage excellente et son procédé de fabrication Expired - Lifetime EP1176217B1 (fr)

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Application Number Priority Date Filing Date Title
JP2000221580 2000-07-24
JP2000221580 2000-07-24

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EP1176217A2 EP1176217A2 (fr) 2002-01-30
EP1176217A3 EP1176217A3 (fr) 2003-04-23
EP1176217B1 true EP1176217B1 (fr) 2011-12-21

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TWI236503B (en) * 2001-10-04 2005-07-21 Nippon Steel Corp High-strength thin steel sheet drawable and excellent in shape fixation property and method of producing the same
JP4502646B2 (ja) * 2004-01-21 2010-07-14 株式会社神戸製鋼所 加工性、疲労特性および表面性状に優れた高強度熱延鋼板
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KR20080110904A (ko) * 2006-05-16 2008-12-19 제이에프이 스틸 가부시키가이샤 신장 특성, 신장 플랜지 특성 및 인장 피로 특성이 우수한 고강도 열연강판 및 그 제조 방법
CN100510143C (zh) * 2006-05-29 2009-07-08 株式会社神户制钢所 延伸凸缘性优异的高强度钢板
JP5040197B2 (ja) 2006-07-10 2012-10-03 Ｊｆｅスチール株式会社 加工性に優れ、かつ熱処理後の強度靭性に優れた熱延薄鋼板およびその製造方法
JP5194858B2 (ja) * 2008-02-08 2013-05-08 Ｊｆｅスチール株式会社 高強度熱延鋼板およびその製造方法
WO2013150669A1 (fr) 2012-04-06 2013-10-10 新日鐵住金株式会社 Tôle d'acier laminée à chaud recuite après galvanisation et son procédé de fabrication
JP5729523B1 (ja) 2013-06-27 2015-06-03 Ｊｆｅスチール株式会社 高強度熱延鋼板およびその製造方法
CN104561831A (zh) * 2015-01-30 2015-04-29 宝山钢铁股份有限公司 一种具有高止裂性能的钢板及其制造方法
KR101957078B1 (ko) 2015-02-20 2019-03-11 신닛테츠스미킨 카부시키카이샤 열연 강판
WO2016132549A1 (fr) 2015-02-20 2016-08-25 新日鐵住金株式会社 Tôle d'acier laminée à chaud
CN107406929B (zh) 2015-02-25 2019-01-04 新日铁住金株式会社 热轧钢板
WO2016135898A1 (fr) 2015-02-25 2016-09-01 新日鐵住金株式会社 Feuille ou plaque d'acier laminée à chaud
CN109563580A (zh) 2016-08-05 2019-04-02 新日铁住金株式会社 钢板及镀覆钢板
CN109563586B (zh) 2016-08-05 2021-02-09 日本制铁株式会社 钢板及镀覆钢板
WO2018138791A1 (fr) * 2017-01-25 2018-08-02 新日鐵住金株式会社 Plaque d'acier

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